Recently, demands in the market include to down size particles diameters of toners for improving image qualities of output images, and to improve low temperature fixing abilities of toners for energy saving.
A toner obtained by the conventional kneading-pulverizing method has irregular shapes with a broad particle size distribution, and it is difficult to obtain smaller particle diameters of a toner by such a method. Moreover, the toner obtained by this method has various problems, including the above, such as high energy requirements for fixing. Especially, during the fixing, the toner produced by the kneading-pulverizing method has a large amount of a releasing agent (wax) present at surfaces of toner particles, as the kneaded product is cracked at the surface of the releasing agent (the wax) by the pulverization to produce the toner particles. For this reason, the releasing effect is enhanced, but the toner tends to deposit on a carrier, a photoconductor, and a blade. Therefore, such the toner does not have satisfactory characteristics on the whole.
In order to solve the aforementioned problems in the kneading-pulverizing method, there has been proposed a production method of a toner by a polymerization method. The toner produced by this polymerization method can be easily made to have small particle diameters, and has a sharper particle size distribution than that of the toner obtained by the pulverization method, and has the wax encapsulated in the toner particles.
As the toner production method by such the polymerization method, there has been proposed a production method of a toner, in which an elongation reaction product of a urethane-modified polyester is used as a toner binder to produce a toner having a practical sphericity of 0.90 to 1.00 for the purpose of improving flowing ability, low temperature fixing ability, and hot offset resistance of the toner (see PTL1).
Moreover, there have been disclosed methods for producing a toner, which has excellent powder flow ability, and transfer ability in the case where particle diameters of the toner are reduced, as well as having excellent heat resistance storage stability, low temperature fixing ability, and hot offset resistance of the toner (see PTL2 and PTL3).
Furthermore, there have been disclosed methods of producing a toner, in which a toner binder having a stable molecular weight distribution is produced, and a maturing step is provided for attaining both low temperature fixing ability and offset resistance of the toner (see PTL4 and PTL5).
There has been also disclosed a method where crystalline polyester is introduced by a polymerization method for improving low temperature fixing ability of a toner. As a preparation method of a crystalline polyester dispersion liquid, for example, PTL6 discloses a preparation method of a dispersion liquid using a solvent for phase separation. By this proposed method, however, only a coarse dispersion liquid having a dispersed particle diameter of several tens micrometers to several hundreds micrometers is obtained. This method cannot yield a dispersion liquid having a volume average particle diameter of 1.0 μm or smaller, which can be used for the production of the toner.
Moreover, in PTL7, reduction of particle diameters of a toner is attempted by mixing crystalline polyester alone into a solvent and heating and cooling the mixture, for the purpose attaining reduced particle diameters of dispersed crystalline polyester in a dispersion liquid. The resulting dispersion liquid is however not stable, which is not satisfactory.
Moreover, PTL8 discloses that crystalline polyester is used, and glass transition temperature thereof before and after thermofusion thereof is controlled in a certain range for attaining both low temperature fixing ability and heat resistance storage stability of a toner. This proposed method, however, does not achieve sufficient low temperature fixing ability of a toner.
Furthermore, PTL9 discloses that a toner having a clear endothermic peak at 50° C. to 100° C. in the process of the first elevation of the temperature, and has the peak area reduced in ⅓ or smaller in the process of the second elevation of the temperature on the differential scanning calorimetry curve of the toner as measured by differential scanning calorimeter (DSC) to achieve low fixing temperature, high transparency of printed images, and reducing sticking of images during double-sided printing. It is, however, does not achieve sufficient low temperature fixing ability of a toner.
The toner production methods proposed in PTL1, PTL2, and PTL3 include a step of increasing molecular weights, in which polyester prepolymer containing an isocyanate group is subjected to a polyaddition reaction with amines in a reaction system where an organic solvent and an aqueous medium are mixed.
In the case of the aforementioned methods and toners obtained by such methods, hot offset resistance of the resulting toner improves, but low temperature fixing ability thereof is degraded, and glossiness of an image after fixing reduces. Therefore, these methods are not yet sufficient enough to solve the problems.
Furthermore, the toner production methods disclosed in PTL4 and PTL5 can be easily employed to a polycondensation reaction, which is a high temperature reaction, but cannot be employed to the aforementioned reaction system where the organic solvent and the aqueous medium are mixed, unless various conditions are optimized.
Although the crystalline polyester resin is introduced by the polymerization method in PTL6 and PTL7 for improving the low temperature fixing ability of the toner, the dispersion liquid having small particle diameters cannot be stably obtained. As a result, undesirable toner particle size distribution is provided, and moreover the crystalline polyester resin is extruded onto surfaced of toner particles, which causes filming. Therefore, these are not sufficient.
Accordingly, it is a current situation that it is desired to promptly provide a toner and a developer containing the toner, having excellent low temperature fixing ability and offset resistance, and capable of forming high quality images with excellent sharpness over a long period without causing filming of a crystalline polyester resin.